Vacuum tube



P, L, SPENCER Feb. 26, 1935.,

VACUUM TUBE Original Filed June 5, 1929 INVENTOR pzreov L. JPEA/CEQ Patented Feb. 25, 1935 -.V.UNITED" stars lenses ATE VACUUM TUBE 6 Claims. '(Cl. 251F275) This invention relates to vacuum tubes and particularly to the construction of the cathodes. Filamentary cathodes as a rule when bent back upon themselves are spread out either to form a 5.hairpin or V'shape, or if too long, an M. If such cathodes are energized by alternatingcurrent the field around each length of filament has substantially the same efiect upon the grid as on the field of the other lengths of cathode. result is that the unneutralized and constantly changing field from each length of filament sets up a hum in the tube which is usually objectionable.' Furthermore, the grid and plate electrodes of a tube having a cathode of this character are generally of oblong cross-section in order to have these electrode surfaces at a substantially constant distance from the various portions of the cathode.

An object of my invention is to devise a cathode 20 which may be energized by alternating current and around which the field is substantially neutralized. A further object is to permit the use of grid and anode electrodes of circular crosssection resulting in tubes having more uniform 25 properties and which are easier and more expeditiously fabricated.

Referring to the drawing, Figure 1 shows a cross-section of a tube embodying my invention. Figure 2 is a view looking down upon the tube 30 showing the circular shape of the electrodes.

Figure 3 is a detailed View of a modification of the cathode.

Within the customary glass envelope 1 is a reentrant stem 2 terminating in a press 3. Sealed in this press are the cathode grid and anode electrodes of the tube shown. The anode consists of a circular nickel member supported by wires 11 and 12 sealed in the press. Wire 12 terminates in a lead 13 for connection outside. concentrically disposed within the anode is a helical wire member comprising a grid 15. This is similarly supported by two wires 16 and 17, the latter terminating in a lead 18. While I have shown the grid and anode as circular, it is obvious that my invention is not to be limited thereto. These electrodes may take on a variety of shapes.

Substantially at the center of the concentric grid and anode is a cathode designated generally by numeral 20. This cathode consists of a length of filament of the ordinary and well known variety such as platinum, nickel alloy or the like. This filament is treated in the well known manner with the result that a coating of chemicals The is obtained on the'surface which promotes. the emission of electrons upon the heating of the filament. .This coating may consists of the oxides arms of alkaline earth metals comprising the well known Wehnelt cathode. This length of fila ment is bent back upon itself-in the manner shown to'formtwo portions so'that the two portions are substantially in contact for their length. The ends ofthe filament are spread out and Welded or secured to supporting wires 25 and 26 terminating in leads 27 and 28. Preferably althoughnot necessarily I dispose a supporting wire 30 sealed in the press so that the bight of the cathode is supported and held taut.

I have found that the coatings on the cathode filament used to promote electron emission are sufiiciently insulating so that the filament heating current will traverse the entire filament strip even when bent in the manner shown. It isdesirable, although not necessary to choose the filament of such alength and cross-section that the necessary voltage to energize it is low, of the order of one or two volts. It is evident that by having the filament disposed in the manner shown the centers of the opposing magnetic fields in the two halves of the filament will be substantially coincident and result in complete neutralization. This, of course, will mean that the tube will not have the customary hum in the output. 7

Furthermore, because of the compact electron emitting surface, circular grid and anode electrodes may be effectively used. Because ofthe symmetry of the construction, less care in the welding of the supporting Wires to the electrodes i necessary and more uniform tubes result.

If desired, the filament may be divided into more than two lengths as shown in Figure 3. In this figure the filament is shown as divided into four portions all'of which, are crushed together into a compact cathode. The filament may be divided into as many sections as may be found desirable. I

I claim:

1. A filamentary cathode for a vacuum tube adapted to be energized by alternating current and having substantially no resultant field around it consisting of a filament upon the surface of which are suitable chemicals for the promotion of electron emission therefrom, said filament being bent into a plurality-of portions, said portions contacting with each other substantially throughout their lengths.

2. A vacuum tube comprising a cathode adapted to be energized by alternating currentwithout having a resultant field around it, said cathode consisting of a length of filament having its surface treated to promote electron emission. and being bent to form a plurality of substantially parallel portions, said portions contacting with each other for substantially their entire length, and a grid and anode surrounding said cathode, both of said latter electrodes being circular in cross-section. a V

3. A method of making a cathode adapted to be energized by alternating current without the formation of a result-ant magnetic field around it, which consists of treating the surface of a metal filament with compounds adapted to promote electron emission therefrom when said filament is heated, dividing said filament into a plurality of portions and bending said filament so as to dispose said portions parallel to each other and crushing the filament to cause said portions to contact with each other substantially along their entire lengths. j v

- 4. A cathode for a thermionic space current tube comprising a, filamentary conducting member bent over itself and contacting along adjacent surfaces, said cathode being arranged to be heated to a temperature ofelectron emission by current from a low voltage source connected to the ends of said filamentary conducting member, the surface of said member having a relatively high contact resistance opposing current flow between adjacent contacting portions of different potential.

5. A cathode for a thermionic space current tube comprising a filamentary conducting member bent over itself and contacting along adjacent surfaces, said cathode being arranged to be heated to a temperature of electron emission by current from a 10W voltage source connected to the ends of said filamentary conducting member, the surface of said member having an electron-emitting coating of relatively high contact resistance opposing current flow between adjacent contacting portions of different potential.

6.'In a vacuum tube, a filamentary cathode adapted to be energized by alternating current and having substantially no resultant field around it, consisting of :a filament upon the surface of which are suitable chemicals for the promotion of electron emission therefrom, said filament being bent into a plurality of serially-connected portions, said portions contacting with each other substantially throughout their lengths.

PERCY .L. SPENCER. 

